The transferrin receptor mediates cellular iron uptake via the iron-binding protein, transferrin, within the hematopoietic system and in other tissues. Monoclonal antibodies against the transferrin receptor can block receptor function and inhibit cell growth. The longterm objective of the proposed work is to understand the structure-function relationship of the transferrin receptor and its role in cell growth. This knowledge will contribute to related practical applications of monoclonal antibodies against the transferrin receptor and growth-related receptors in cancer and more effective methods of antibody-mediated drug targeting. As a continuation of previous structural and functional studies, it is proposed to prepare monoclonal antibodies of predetermined specificity that react with the human transferrin receptor to further define how antibodies block receptor function. Antibodies against the cytoplasmic domain of the transferrin receptor will be obtained and used to purify the intracellular membrane compartment enriched in transferrin receptors in order to learn more about the molecular basis of receptor trafficking within cells. As complementary approaches to understanding transferrin receptor function, in vitro mutagenesis will be used to modify the human transferrin receptor and the structural and functional consequences of the mutations analyzed, and efforts will be made to obtain sufficient amounts of transferrin receptor either in a bacterial expression system or by overexpression in mammalian cells to initiate X-ray crystallographic studies. Collaborative studies are proposed to characterize mutant mouse lymphoma cells resistant to killing by anti-transferrin receptor antibodies and ricin A-antibody conjugates and to investigate the regulation of transferrin receptor expression during the growth and differentiation of HL-60 cells.